Documentation and symbolic constants for dispatcher actions (#31278)

* Remove unreachable code

`view_last_screen` is never used.

* Remove unused action `user_activity_started`

Nothing listens to this, so it's pointless.

* Symbolic constant for `Action.UserActivity`

* Define symbolic constants for more `Action`s

Constants for some actions that are emitted by `Lifecycle`

src/vector/platform/ElectronPlatform.tsx

@@ -218,7 +218,7 @@ export default class ElectronPlatform extends BasePlatform {
             this.electron.send("app_onAction", payload);
         }
 
-        if (payload.action === "client_started") {
+        if (payload.action === Action.ClientStarted) {
             this.clientStartedPromiseWithResolvers.resolve();
         }
     }

src/vector/platform/WebPlatform.ts

@@ -53,7 +53,7 @@ export default class WebPlatform extends BasePlatform {
         super.onAction(payload);
 
         switch (payload.action) {
-            case "client_started":
+            case Action.ClientStarted:
                 // Defer drawing the toast until the client is started as the lifecycle methods reset the ToastStore right before
                 this.registerServiceWorkerPromise.catch(this.handleServiceWorkerRegistrationError);
                 break;